The invention concerns mineral processing to recover precious metals, including recovery of gold following cementation of the gold out of leach solutions.
Precious metals, especially gold, are frequently recovered from precious metal-containing ores, concentrate, and other precious metal-containing mineral materials by leaching the precious metal into a leach solution including a lixiviant for the precious metal. Examples of lixiviants used to leach gold include certain cyanide salts, thiosulfate salts and thiorea. The gold may be recovered from the pregnant leach solution by a variety of techniques, depending upon the lixiviant that is used. For example, a common technique for recovering gold from a cyanide leach solution is to adsorb the gold-cyanide complex onto activated carbon granules, remove the gold-loaded carbon granules from the leach solution and strip the gold off of the granules using a strip solution. Another technique is to contact the pregnant leach solution with an ion exchange resin capable of removing the precious metal from the leach solution.
Yet another technique for recovering precious metals from a pregnant leach solution is cementation. In cementation, pieces of another metal, typically in a particulate form, such as in the form of a powder, granules, or beads, is contacted with the pregnant leach solution under conditions so that some of the other metal dissolves into the leach solution and displaces dissolved precious metal from the solution. Precious metal displaced from the solution deposits on the pieces of the other metal to form a cementation product including a thin precious metal-containing coating supported on the pieces of the other metal. The pieces of the other metal, therefore, act as a substrate on which a coating of the precious metal deposits during the cementation. The other metal, or substrate metal, is typically a base metal, and it is important that the electrode potential between the substrate metal and the precious metal be large enough to adequately drive the cementation reaction. For example, zinc works well as a substrate metal for cementation of gold from cyanide leach solutions and copper works well as a substrate metal for cementation of gold from thiosulfate leach solutions.
One problem with cementation is that it can be expensive to subsequently separate the precious metal from the other metal. The substrate metal, which typically makes up a much larger portion of the cementation product than the precious metal, is a nuisance in the smelting operation and increases the cost of preparing a purified precious metal product. This is one reason why it is often preferred to find an alternative technique for recovering gold from leach solutions. In the case of gold recovery using cyanide leach solutions, problems associated with cementation are typically avoided by removing gold from the leach solution using activated carbon.
In the case of thiosulfate lixiviants, activated carbon is not effective for removing gold from the thiosulfate leach solution, and the use of ion exchange resins is expensive. Cementation, particularly on copper, has been found effective for removing gold from thiosulfate leach solutions, but the copper is a nuisance during smelting and refining operations. To increase the surface area available for cementation and thereby also increase gold loading per unit weight of copper, a fine copper powder has been used for cementation of gold. One problem with using a fine copper powder, however, is that it is difficult to adequately clarify the pregnant thiosulfate leach solution prior to cementation. Very fine filtration is typically required, which is expensive. Also, even when using a fine copper powder, a large quantity of copper must be processed during subsequent smelting and refining operations, significantly adding to the ultimate cost of preparing a purified gold product. This is in addition to the cost of the copper that is consumed. One solution to the clarification problem would be to use relatively large copper beads that could be easily separated from the leach solution by simple screening. This has the effect, however, of significantly increasing copper consumption and also the amount of nuisance copper that must be processed during precious metal smelting and refining operations.
There is a significant need for improved cementation operations for recovering precious metals, and especially for recovering gold from thiosulfate leach solutions, that permit easier clarification of the leach solution, and/or that reduce the quantity of the substrate metal from the cementation operation that must be processed along with the precious metal during smelting and refining operations, and/or that reduce the quantity of the other metal consumed per unit weight of precious metal recovered.
It has been found with the present invention that the amount of the substrate consumed to recover precious metals by cementation from thiosulfate leach solutions can be significantly reduced, and also the quantity of the substrate metal that must be processed during smelting and refining operations can be significantly reduced, by selectively dissolving from the cementation product a small portion of the substrate metal thereby effecting physical release of the precious metal from the substrate. The released precious metal can then be separated from the substrate for subsequent smelting and refining operations to prepare a purified precious metal product. In this way, the amount of substrate metal (e.g., copper or zinc) that needs to be processed during refining operations is significantly reduced. The separated substrate particles can then be recycled for cementation of additional precious metal, with a result being that less of the substrate metal is consumed per unit weight of precious metal recovered. An additional advantage is that it is possible to use relatively large, or coarse, particles of the substrate metal for the cementation, which significantly simplifies clarification of the leach solution because the larger particles are easier to remove than a fine powder. A simple screen is typically adequate for separating the cementation product from the barren leach solution following cementation. The use of relatively large particles of the substrate metal is possible because only a small amount of substrate metal is consumed during the selective dissolution to release the precious metal, permitting particles of the substrate metal to be reused several times for cementation of the precious metal.
In one aspect, the present invention provides a method for removing a precious metal from a cementation product including a substrate loaded with a precious metal-containing coating. The method involves selectively dissolving into a dissolution solution only a portion of the substrate to effect physical release of at least a portion of the precious metal-containing coating. Preferably only a small portion of the substrate is dissolved, while only a negligible amount or none of the precious metal is dissolved. The released precious metal, which remains in a solid form, can then be separated from the dissolution solution for further processing.
In one specific embodiment, the particulate substrate is a particulate base metal and the precious metal-containing coating comprises gold that has been loaded onto the particulate base metal by cementation. When cementation of the gold is from a thiosulfate leach solution, the particulate substrate will preferably be a particulate copper. When cementation of the gold is from a cyanide leach solution, the particulate substrate will preferably be particulate zinc. When the particulate substrate is a base metal, and particularly when the particulate substrate is copper or zinc, a preferred dissolution solution is an ammonium carbonate solution. In most instances,: it is necessary to dissolve only a few percent or less of the particulate substrate to effect release of the gold, and the particulate substrate can be reused many times for further cementation. As separated from the dissolution solution, the gold is ordinarily in fine particulate sludge that may be about 3 times or more concentrated in gold than current typical cementation product produced using a fine base metal particulate. In one embodiment of the present invention, base metal components of the fine particulate sludge can be selectively dissolved to produce a product with an even higher gold content. Smelting and refining can be expected to be significantly less expensive for processing this more concentrated product than current conventional cementation product.
In another aspect, the present invention provides a method for recovering gold from a gold-containing mineral material that involves leaching the mineral material to dissolve at least a portion of the gold into the leach solution to form a pregnant leach solution, followed by contacting the pregnant leach solution with a particulate substrate in a manner to remove at least a portion of the gold from the pregnant leach solution and load gold onto the particulate substrate, such as by cementation. The gold-loaded particulate substrate is then separated from the leach solution and contacted with a dissolution solution to dissolve a portion, and preferably only a small portion, of the particulate substrate, so that at least a portion of a gold-containing coating is physically released from the particulate substrate in the form of a fine particulate. The particulate substrate can then be separated from the dissolution solution and the fine particulate, such as by screening out the coarser particulate substrate. The precious metal-containing fine particulate can then be separated from the dissolution solution, such as by filtration.
These and other aspects of the invention are further described below. Also, although the invention is described primarily with respect to recovery of gold, the same principles apply to recovery of other precious metals in operations in which the precious metal is coated on a particulate substrate, so long as a portion of the substrate material is selectively dissolvable in a manner to physically release the precious metal in solid form from the particulate substrate. Furthermore, the invention is described primarily with reference to removal of precious metal from a cementation product but in a broad sense the invention is not so limited and includes the processing of any precious metal-containing material having a substrate and a precious metal-containing surface coating supported on the substrate, wherein a small portion of the substrate is selectively dissolvable to physically release precious metal-containing coating, and especially when the substrate is a base metal material.